In a wireless communication system having a wireless base station (BS: Base Station) and a wireless mobile station (MS: Mobile Station), the MS can communicate with another MS, a wire terminal, a server or the like via the BS.
As one of the above wireless communication systems, there is WiMAX system in conformity with a system called WiMAX (World wide Interoperability for Microwave Access) that is under development in these years. The WiMAX system employs Orthogonal Frequency Division Multiplexing Access (OFDMA) and Adaptive Modulation and Coding (AMC) in order to improve the communication efficiency.
In a wireless frame used in WiMAX systems, a subframe (DL subframe) in the downlink (DL) in a direction from a BS to a MS and a subframe (UL subframe) in the uplink (UL) in a direction from the MS to the BS are time-division-multiplexed.
In the DL subframe, preamble, frame control header (FCH), map information in DL (DL map), map information in UL (UL map) (hereinafter, these will be occasionally referred to as header information) and one or more DL bursts are multiplexed in a two dimensional region expressed by time axis (symbol time) and frequency axis (subchannel frequency). In the UL sub frame, one or more UL bursts are multiplexed. Incidentally, “burst” signifies a wireless resource (communication region) specified by subchannel frequency and time (transmission timing).
Preamble is a region (field) into which frame synchronization information is inserted. FCH is a region into which information defining map information such as size, position, etc. of the map information is inserted. Map information includes information about ID (CID) of a communication connection to be transmitted in a burst, arranged position (burst position) of the burst of the connection in the wireless frame, size of the burst (burst size), modulation scheme of the burst (QPSK, 16QAM, 64QAM, etc.) and coding rate of the burst, transmission power control information (boost up/down), etc.
Namely, it can be said that the map information is information (burst allocation information) that specifies (allocates) regions (reception region and transmission region) of a wireless frame to be received by or transmitted from the MS. The burst position can be specified by a symbol offset from the leading symbol and a subchannel offset. The burst size can be specified by a symbol number and a subchannel number.
Accordingly, the MS can establish wireless frame synchronization of DL and UL by detecting the preamble, demodulate and decode a DL burst region specified by the DL map in a demodulation scheme and at a decoding rate corresponding to a modulation scheme, a coding rate, etc. specified by the DL map, thereby to selectively receive the DL burst destined for the MS. The MS can transmit data to the BS in the UL burst region specified by the UL map.
Further, the MS can measure communication quality (reception quality) in a wireless section between the MS and the BS and report, regularly or irregularly, the result of the measurement (reception quality information such as SINR: Signal to Interference and Noise Ratio, etc. for example) to the BS. The BS can thereby adaptively control the modulation scheme and coding rate in each burst on the basis of the reception quality information from the MS. This is called AMC (Adaptive Modulation and Coding).
As a power allocation method in OFDMA systems, a method is described in the following patent document 1 below, in which a transmission power level for each wireless unit is selected in response to CQI of the wireless unit.
Moreover, in the patent document 2 below, there is described a method as a method for allocating subcarriers to subchannels to be allocated to at least one user in a multi user communication system using a plurality of subcarriers, in which the quality of each subchannel is detected and a subchannel is allocated to the subcarrier according to the quality level.    Patent Document 1: Japanese Patent Publication No. 2004-129241    Patent Document 2: Japanese Patent Publication No. 2006-50615
Now, the above-described AMC control will be described with reference to FIGS. 7 and 8. FIG. 7 is a diagram illustrating a throughput characteristic of DL obtained in each modulation scheme and at each coding rate. FIG. 8 is a diagram illustrating an example of the AMC control.
As illustrated in FIG. 7, the relationship between SINR and throughput is a stage-like relationship when SINR is represented by the horizontal axis and the throughput is represented by the vertical axis, and a suitable modulation scheme and coding rate exist according to communication environments (the above-mentioned reception quality information at the MS) in a wireless section between the MS and the BS. For example, as illustrated in FIG. 8, when SINR (dB) which is the reception quality information from the MS is a value not less than A and below B, the BS selects 16QAM (½) (figure in parenthesis representing coding rate) illustrating a throughput (kbps) characteristic designated by a symbol a in FIG. 7.
Likewise, when SINR from the MS is a value not less than B and below C in FIG. 8, the BS selects 16QAM (¾) illustrating a throughput characteristic designated by a symbol b in FIG. 7. When SINR from the MS is a value not less than C in FIG. 8, the BS selects 64QAM (⅔) illustrating a throughput characteristic designated by a symbol c in FIG. 7.
In this way, the BS selects a suitable modulation scheme and coding rate according to communication environments between the BS and the MS to transmit data to the MS.
When attention is given to an MS that has reported SINR close to B, which is the upper limitation value of a range of A≦SINR<B, among MSs that have reported SINRs in this range illustrated in FIG. 8, a little improvement in the SINR allows the modulation scheme (coding rate) to be selected by the BS to step up from 16QAM (½) to 16QAM (¾), which allows a large increase in throughput.
Likewise, when an MS that has reported an SINR close to C, which is the upper limitation value of a range of B≦SINR<C in FIGS. 7 and 8, among MSs that have reported SINRs, a little improvement in the SINR allows a modulation scheme (coding rate) to be selected by the BS to step up to 64QAM (⅔) from 16QAM (¾), which allows a large increase in throughput.
Therefore, if an SINR measured by an MS that has reported the SINR in the vicinity of a border across which the modulation scheme (coding rate) to be selected by the MS is changed as above is improved, a large increase in throughput can be expected.
Since the reception SINR at the MS can be improved by increasing the transmission power from the BS, the transmission power of an DL burst destined for the MS is increased (boosted up) on the BS's side.
However, the following problem will occur when the above transmission power increase control is realized.
Namely, upper limitation of a sum of transmission powers in the frequency axial (subcarrier frequency) direction in a certain symbol (time) in a wireless frame is set by the law, limitation of the transmission amplifier, etc. Therefore, when bursts are allocated so that all subcarriers in the frequency axial direction are used in a certain symbol, and, hence, the transmission power reaches the upper limitation, the above transmission power increase control is not executable.